<style>
  * {
    padding: 0;
    margin: 0;
  }

  body {
    background: #2c3e50;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    box-sizing: border-box;
    width: 100vw;
    height: 100vh;
    padding: 80px;
  }

  main {
    width: 400px;
    height: 400px;
  }

  div {
    width: 150px;
    height: 150px;
    background-color: #fff;
    border: solid 20px #ddd;
    /* animation-name: bigger;
    animation-duration: 3s; */
    animation: bigger 3s ease-in-out infinite;
  }

  @keyframes bigger {
    0% {
      opacity: 0;
      transform: scale(.1);
    }

    50% {
      opacity: 1;
      transform: scale(1);

    }

    100% {
      opacity: 0;
      transform: scale(.1);
    }

  }
</style>

<main>
  <div></div>
</main>